Surface-acouustic-wave elements were originally limited to a specific military use. Recently, however, their use has been developed to FM tuners, television systems and other civilian devices, and a great importance is placed on them. Surface-acoustic-wave-elements are practically used as delay elements, oscillators, filters or other electronic elements. Advantageous features of various surface-acoustic-wave elements are miniatured sizes, decreased weights, high reliabilities and availability of mass-production due to their manufacturing process similar to that of integrated circuits. Under the circumstances, a great deal of surface-acoustic-wave elements are produced as indispensable electronic parts.
FIG. 4 shows one form of a surface-acoustic-wave resonator used in an oscillator or other devices.
The surface-acoustic-wave resonator includes a piezoelectric substrate 1 along which a surface acoustic wave travels, an interdigital electrode 2 provided on the piezoelectric substrate 1 to excite a surface acoustic wave, and comb-shaped reflectors 3 and 3' having a number of metal strips aligned at an interval and extending across the wave traveling direction.
When a voltage of a specific frequency is applied to the interdigital electrode, an electric field is produced on the surface of the piezoelectric substrate between electrode fingers of the interdigital electrode 2. Due to a piezoelectric property of the substrate 1, a distortion proportional to the voltage occurs and expands as a surface acoustic wave in both directions at a speed determined by the material of the piezoelectric substrate 1. The surface acoustic wave is reflected by the comb-shaped reflectors 3 and 3' to the interdigital electrode 2 and exhibits a resonance.
The resonator of FIG. 4 includes open-type reflectors in which metal strips are not connected to each other. However, some surface-acoustic-wave resonators as shown in FIG. 5 includes short-circuited reflectors 3 and 3' in which metal strips are connected to each other at respective ends thereof. Impedance properties of the surface-acoustic-wave resonators are shown by a reflection constant chart of FIG. 6.
In the same drawing, an upper semicircle A is a region in which the surface-acoustic-wave resonator exhibits inductive properties, and a lower semicircle B is a region in which the surface-acoustic-wave element exhibits capacitive properties. A curve shown by a solid line is a vector orbit obtained by applying a voltage of a desired frequency to the interdigital electrode 2. Capital letter C indicates a resonance point, D shows an anti-resonance point, M is a major resonance area, and S is a sub-resonance area. The outer circle is a circle of absolute value 1 of the reflection constant.
The drawing shows that when the frequency .omega. of a voltage applied to the interdigital electrode 2 is increased from a frequency lower than the resonance frequency to a frequency higher than the resonance frequency, the resonator exhibits inductance properties at two areas (shown by hatchings). More specifically, one of the areas is a major resonance area M produced by reflected waves from the reflectors 3 and 3' whereas the other area is a sub-resonance area S produced by internal reflected waves in the interdigital electrode 2.
In presence of two or more inductive frequency areas in a single resonator used in a Colpitts oscillator to excite a predetermined frequency, the resonator sometimes excites a frequency other than the predetermined value due to an external disturbance or other external factor because the inductive areas are used for excitation.